Refactor and bug fix for uReplicator Worker

[yangy0000]

1. parallel commit offset
2. fix worker failed to commit offset problem for federated mode on route re-assign
3. refactor on worker federated mode 3.1. throw exception on unexpected behaviour 3.2. add more check to avoid potential race condition 3.3. add unit test for error scenarios
4. not to shutdown fetcher thread when the assignment is empty